1. Technical Field of the Invention
The present invention is related to a micro-electro-mechanical device. In particular, the present invention relates to a micro-electro-mechanical device with a suspended structure formed from mono-crystalline silicon, bonded to a substrate wafer with an organic adhesive layer. Still more particularly, the present invention has part of said organic adhesive layer serving as support and spacer and the rest of said organic adhesive layer serving as a sacrificial layer, which is removed by a dry etch means. Even more particularly, the substrate wafer may contain integrated circuits for sensing and controlling said device.
2. Description of the Related Art
A micro-electro-mechanical system (MEMS) is a device that transforms mechanical activity into electrical signals and vice versa. A key element in MEMS is a suspended microstructure, which deflects in response to applied force to the microstructure or to a proof mass attached to it. The amount of deflection can be sensed electrically by capacitive plates coupled to surfaces of both the suspended mass and an adjacent structure or by piezoelectric or piezo-resistive layers formed on the microstructure. As the suspended mass deflects, the resulting change in capacitance or voltage from the sensing elements provides an electrical indication of the applied force. One example is an accelerometer that comprises a rectangular membrane, one side of which is fixed to a carrier while the other is suspended, and a means for detecting the movement of the membrane under the effect of acceleration. This constitutes a force sensor. Conversely, an electrical signal applied to the capacitive plates or piezoelectric layers can result in deflection in the suspended mass. This constitutes an actuator.
There are two major classifications of methods for making thin suspended microstructures: (1) Bulk micro-machining, where the transducers are primarily shaped by etching a thick mono-crystalline silicon substrate; and (2) Surface micro-machining, where transducers are constructed entirely from thin films deposited on respective sacrificial layers. Mono-crystalline silicon used in bulk micro-machining has two major benefits. The first benefit is the silicon is almost a perfect mechanical material. It's stronger than steel by weight; does not show mechanical hysteresis and is highly sensitive to stress. This technology requires a deep silicon etch to remove the bulk of the material to form the microstructures. This is normally by anisotropic wet etch where both dimension control and contamination control is a major challenge. An important feature of bulk micro-machining is the microstructure is often bonded to another wafer for packaging. The bonding techniques include anodic bonding, metallic seals, and low-temperature glass bonding. Polymer bonding is attractive because it is widely used in attaching semiconductor dies to substrates for packaging, but it is rarely used in micro-machining as most these applications require very thin bondlines. The application of a thin layer of polymer adhesive requires thinning with solvent that is incompatible with most micro-machining techniques.
Surface micro-machined MEMS devices are constructed entirely from thin films deposited on a sacrificial layer. These devices allow for monolithic integration with silicon processors using the standard silicon process technology commonly known to individuals skilled in the art. The sacrificial layer is either made of organic polymer such as photoresist or inorganic substance such as silicon oxide. Photoresist can be dry etched in oxygen plasma but can not withstand high temperature anneal like silicon dioxide can. Chemical-vapor deposited polysilicon film is used in integrated accelerometers and many other MEMS devices. Unfortunately, it must be annealed at a high temperature (.about.1000.degree. C.) to reduce stress, or its suspended structure will curl after the sacrificial layer is removed from underneath. The sacrificial layer is made of silicon dioxide whose removal is by wet etch in HF acid. Surface tension of the aqueous HF acid solution exerts forces on the suspended microstructure, which pulls the microstructure into contact with the substrate and causes them to stick together. To separate them without causing damages is difficult because the combination of adhesive forces and electrostatic forces is large compared to the strength of the thin films. Another drawback is the metal interconnect in the integrated control circuits can not withstand the high temperature anneal. Therefore, it must be formed after the polysilicon suspended microstructure is formed. Protecting the suspended microstructure during interconnect process and the wet etch is a complex matter and entails the usage of costly state-of-the-art fabrication facilities.
Surface micro-machining that does not require high temperature anneal has distinct advantages because dry etch can be used for removing the sacrificial layer and the microstructure can be fabricated on finished IC's. This avoids the sticking associated with the wet etch process and the expensive equipment thereby necessitated. However, the intrinsic stress and hysteresis in the deposited film limits its thickness to a few thousand angstroms or the films can curl and change after stressing. This makes the technique not suitable for devices such as accelerometers that require larger proof mass.
Therefore, it is highly desirable to combine the merits of both bulk and surface micro-machining techniques--such as MEMS devices employing suspended structure made from mono-crystalline silicon by surface micro-machining method. One example is surface micro-machined accelerometers made with silicon-on-insulator (SOI) wafers, wherein thick mono-crystalline silicon is bonded to another silicon wafer with silicon oxide insulator in between. The suspended structure can be made much thicker than mono-crystalline for increased proof mass. However, SOI wafers are much more expensive than the regular wafers and its production employs a high temperature process that is not compatible with integrated circuits and dry-etchable sacrificial layers. It would be of great advantage if the sacrificial layer were dry-etchable like organic polymers. Hence, there is a strong need in the industry to overcome the aforementioned shortcomings of the present art.